Writing fluids, i.e. various kinds of inks, are known containing gelatin breakdown products as dispersion stabilizers; for instance as described in Chemical Abstracts 101 (22), 193881y (1984) and Chemical Abstracts 102 (8), 63782b (1985). For the production of such inks, it has been proposed to employ a gelatin glue solution which has been degraded by acid hydrolysis. In order to produce such a glue solution, commercial gelatin is treated in an aqueous solution at an elevated temperature with concentrated sulfuric acid. After the acid hydrolysis of the gelatin, the acid must be removed, which is done by adding successive portions of barium hydroxide, bringing about precipitation of barium sulfate. The precipitated barium sulfate is removed from the solution by filtration. The filtrate, a low viscosity gum solution, must then be concentrated by the expenditure of considerable energy, in order to bring the active ingredient up to the concentration required for efficacy in ink formulations.
The above-described process exhibits many disadvantages. Thus, the starting solution must be relatively dilute or low in viscosity, in order for the sulfuric acid to bring about the desired degree of degradation. At higher viscosities, the poor dispersion of the sulfuric acid leads to damage to the gelatin molecule, which has an adverse effect on the properties of the ink. The handling of sulfuric acid, especially concentrated sulfuric acid, requires specially stringent safety precautions. The introduction of barium hydroxide is also a matter of toxicological concern. The precipitated barium sulfate must be removed so completely that no toxicologically-significant amount of this by-product appears in the final ink. The disposal of barium sulfate, contaminated with adhering amounts of gelatin breakdown products, can be costly and troublesome.
In order to obtain filterable crystals of the precipitated barium sulfate, it is necessary to use well-controlled process conditions. In this regard, it is especially necessary for the crystallization process to run for a long period of time, in order to obtain the desired crystal size. Therefore, the barium hydroxide addition must be done at an inconveniently slow rate. Further problems arise in connection with disposal of the barium sulfate, which because of its toxicity must be placed in special toxic waste dumps. Moreover, there is not only the necessity for precipitating the sulfuric acid quantitatively but also the necessity to avoid an excess of barium hydroxide, which is also a matter of toxicological concern.